Ordinal relationships exert a pervasive influence on behavior. In the laboratory, ordinal knowledge has been studied in a variety of experiments on both humans and animals. Examples include transitive inference (TI) based on reward contingency or social dominance, knowledge of ordinal relationships between items trained by the simultaneous chaining (SC) paradigm, and extrapolations of ascending and descending rules in the case of numerical stimuli. RT and accuracy data from these studies reveal very similar distance and magnitude effects in humans and animals, implying that they use similar nonverbal strategies. The results of these experiments have opened a new line of research on serial learning that has the potential for defining non-linguistic primitives that are shared by animals and humans. The goal of the proposed research is to explore the concept of an ordinal comparator that represents and integrates ordinal information from different ordinal continua. Two basic features of the ordinal comparator are (1) it does not make any assumptions about linguistic ability and (2) it integrates existing data on ordinal knowledge. Our experiments, which will be performed on rhesus macaques (Macaca mulatta) and adult humans, will use a variety of proven methods (simultaneous chaining, transitive inference, numerical, psychophysical, and spatial discrimination tasks) for training monkeys to execute lists composed of arbitrary stimuli and by stimuli that can be characterized as inherently ordered. On an SC, arbitrary list items (usually photographs) are presented simultaneously on a touch sensitive video monitor. The spatial positions of items within an SC are randomized to minimize spatial and motor cues, except in the case of spatial tasks, in which they are intended to convey ordinal information. On an SC, subjects must learn to generate a representation of the entire sequence and their current position in that sequence, as they move from item to item. Under the TI paradigm, the subject is rewarded each time it chooses an arbitrarily defined correct item from pairs of arbitrarily selected stimuli. Subjects are then tested on their ability to infer the correct item from pairs of non-adjacent items. Numerical stimuli will be trained by conceptual and symbolic matching-to- sample paradigms. Our major hypothesis is that positive transfer will occur between two ordinal tasks, whether spatial, numerical, psychophysical, serial or transitive, to the extent that the representations that mediate each task are isomorphic. Toward that end, we propose 4 lines of comparative research on the non-verbal representation of ordinal distance and numerical magnitude. The proposed research has important interdisciplinary ramifications for studies of neuroscience (neural correlates of ordinal memory), child development (nonverbal tests for autistic children), and mental health (cognitive tests of ordinal memory during electroconvulsive therapy (ECT) and magnetic seizure therapy (MST)).The proposed research will advance understanding of the representation of order in humans and monkeys. This line of research is already being applied directly to studies of anterograde and retrograde amnesia induced by ECT and TMS. In addition we expect the results to have implications for the development of new diagnostic tools for autism spectrum disorder.